Insoluble anode for electrolysis and a method for its production

ABSTRACT

An insoluble anode suitable for use in electrolysis and a method for its production are provided. The anode comprises three components which are: A. A BASE SUBSTRATE TREATED BY SANDBLASTING, SAID SUBSTRATE BEING SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND TANTALUM OR ALLOYS THEREOF; B. A THIN METAL LAYER FORMED ON SAID SUBSTRATE, SAID THIN METAL LAYER BEING SELECTED FROM THE GROUP CONSISTING OF PLATINUM, PALLADIUM AND RHODIUM OR ALLOYS THEREOF; AND C. AN OUTERMOST LAYER OF MANGANESE DIOXIDE ELECTROPLATED ON SAID THIN METAL LAYER IN AN AQUEOUS SOLUTION OF MANGANESE SULFATE CONTAINING ABOUT 10 TO ABOUT 30 GRAMS/LITER OF FREE SULFURIC ACID AT A CURRENT DENSITY OF ABOUT 0.015 TO ABOUT 0.007 A./dm.2. The anode is produced by sandblasting a base substrate metal of titanium or its alloy to form minute indentations on the surface, electroplating on said base substrate metal a thin layer of platinum, and electrodepositing a layer of manganese dioxide on said thin layer in an aqueous solution of manganese sulfate containing 10-30 g./l. of free sulfuric acid at a current density of 0.015-0.007 A./dm.2.

United States atent [72] Inventors Kazuya Osawa Yokohama; Keiichi Shimizu, Tanashi; Takashi Takasue, Tokyo, all of Japan [21] Appl. No. 707,408 [22] Filed Feb. 23, 1968 [45] Patented Oct. 26, 1971 [73] Assignee The Furerkawa Electric Company Limited Tokyo, Japan [32] Priorities Feb. 27, 1967 J p [31] 42/ 12427 Nov. 6. 1967, Japan, No. 42/71334 [54] INSOLUBLE ANODE FOR ELECTROLYSIS AND A METHOD FOR ITS PRODUCTION 4 Claims, No Drawings [52] U.S. Cl 204/40, 204/290 F [51] Int. Cl C23b 5/46 [50] Field of Search 204/290, 290 F, 96,1051, 48,40, 56

[56] References Cited UNITED STATES PATENTS 2,631,115 3/1953 Fox 204/190 Primary Examiner-Howard S. Williams Assistant Examiner-Sidney S. Kanter Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: An insoluble anode suitable for use in electrolysis and a method for its production are provided. The anode comprises three components which are:

a. a base substrate treated by sandblasting, said substrate being selected from the group consisting of titanium. zirconium and tantalum or alloys thereof;

b. a thin metal layer formed on said substrate. said thin metal layer being selected from the group consisting of platinum, palladium and rhodium or alloys thereof: and

c. an outermost layer of manganese dioxide electroplated on said thin metal layer in an aqueous solution of manganese sulfate containing about 10 to about 30 grams/ liter of free sulfuric acid at a current density of about 0.015 to about 0.007 A./dm.-.

The anode is produced by sandblasting a base substrate metal of titanium or its alloy to form minute indentations on the surface. electroplating on said base substrate metal a thin layer of platinum, and electrodepositing a layer of manganese dioxide on said thin layer in an aqueous solution of manganese sulfate containing 10-30 g./l. of free sulfuric acid at a current density of 0.015-0.007 A./dm.

INSOLUBLE ANODE FOR ELECTROLYSIS AND A METHOD FOR ITS PRODUCTION The present invention relates to an insoluble anode for use in various kinds of electrolysis.

The principal object of the invention is to provide an insoluble anode having such characteristics that it is less soluble in an electrolyte thus preventing the electrolyte from pollution and is not deposited on a cathode even when it is desolved in the electrolyte, that it has a low oxygen overvoltage during the electrolysis thus economizing the electric power necessary for the electrolysis, and that it has excellent anode characteristics for electrolytic winning ofzinc.

In the electrolytic industry it is the most important problem to select a suitable insoluble anode having such improved characteristics that it does not pollute an electrolyte, and that it has a long life and a low oxygen overvoltage during the electrolysis. It is known that platinum is an excellent anode material which satisfies the above-mentioned characteristics. However, platinum is expensive and hence is not suitable for industrial use, and thus carbon and lead alloy electrodes have generally been used. The carbon anode, however, has the disadvantage that it greatly pollutes the electrolyte, wears fast, and has a high electric resistance which results in an increase in cell voltage. The disadvantage of the lead alloy anode consists in that lead dissolves into the electrolyte and the solute is deposited on the cathode to decrease the purity of the deposit obtained, that the oxygen overvoltage becomes high, and that in case of electrolytic winning of zinc the zinc obtained contains a considerable amount of lead as an impurity. The zinc obtained is less expensive, but it cannot be used as a diecasting material which requires pure zinc.

Under the above circumstances, it has recently been proposed to electroplate the surface of a titanium substrate with platinum and electrodeposit on the electroplated surface with lead dioxide to manufacture an insoluble anode. This anode improved the characteristics somewhat, but has the disadvantage that it has a comparatively high oxygen overvoltage, that lead dioxide is dissolved into certain electrolyte, to soil it, that lead reduced in the electrolyte is deposited together with electrolytic products on the cathode, and that in case of electrolytic winning ofzinc, the zinc obtained contains lead as an impurity in the same manner as the aforementioned lead alloy anode.

The invention is based on the recognition that electrodeposited manganese dioxide is an insoluble and electrically conductive oxide which, even when dissolved into an electrolyte, cannot easily be deposited as a reduced product on the cathode. Experiments have yielded such an insoluble anode for use in electrolysis that it is less soluble into an electrolyte and hence does not pollute it, that it is not deposited on a cathode even when dissolved into the electrolyte, that it has a low oxygen overvoltage during the electrolysis to economize the electric power necessary for electrolysis, and that it has an optimum effect when applied to electrolytic winning ofzinc.

The invention provides the above-mentioned insoluble anode for use in electrolysis which comprises a base substrate metal of titanium, zirconium, tantalum or an alloy mainly composed thereof sandblasted on the surface, a thin layer of platinum palladium, rhodium, or an alloy mainly composed thereof formed on said substrate metal, and an outermost layer of manganese dioxide electrodeposited on said thin layer.

The electrodeposited manganese dioxide is an insoluble and electrically conductive oxide, while manganese cannot easily be deposited from an aqueous solution and hence gives no disturbance to electrolytic production of zinc. Thus, if use is made of manganese dioxide to produce an insoluble anode for use in electrolysis, it is quite possible to prevent impurities from becoming mixed into an electrolytic product.

The electrodeposited manganese dioxide, however, is very brittle so that it is difficult to form the anode from manganese dioxide itself. Moreover, the electrodeposited manganese dioxide has a large internal stress and is thus liable to be detached from the substrate when it is electrodeposited thereon to form an anode. Any conventional manganese dioxide electrodeposited on the base substrate metal not only dissolves considerable in an electrolyte, but also causes formation of a passive oxide film on the interface between itself and the substrate metal, and therefore cannot be used for an anode.

The invention provides the possibility of applying the electrodeposited manganese dioxide to an anode without causing the above-mentioned unfavorable properties as a result of various investigations. That is, the invention makes use of a base substrate metal of titanium, zirconium, tantalum, or an alloy mainly composed thereof, sandblasted on the surface, In this case, any metal may be alloyed with titanium, zirconium, or tantalum insofar as it does not adversely affect their corrosion resistance. For example, the alloy of titanium, zorconium, and tantalum or one composed of any of these three metals plus a few percent of palladium or aluminum may be used. This base substrate metal of the invention is capable of decreasing the internal stress of deposits fon'ned on it and electrodepositing the manganese dioxide without causing the above-mentioned unfavorable properties.

The insoluble anode of the invention further comprises a thin layer of platinum, palladium, rhodium, or an alloy mainly composed thereof, formed on the aforementioned base substrate metal. In this case, any metal may be alloyed with platinum, palladium, or rhodium to make the thin layer insofar as it is free from oxidation, for example, the alloy of these three metals. The assembly thus formed renders it possible to prevent formation of a passive oxide film on the interface between the base substrate metal and the electrodeposited manganese dioxide layer formed on the thin alloy layer.

The thickness of the thin layer must be so determined that the minute indentations produced by the sandblast will remain discernible. It is preferable to determine the thickness of the thin alloy layer as 0.0 1-0.3 1..

The invention preferably makes use of titanium or its alloy as the alloy substrate material when the anode of the invention is used for the electrolytic winning of zinc. It is preferable to form a thin layer of platinum or its alloy having a thickness of 0.0l-0.3 p. on the interface between the base substrate metal and the electrodeposited manganese dioxide layer. The reason is that the thickness of less than 0.01 p. is less effective in preventing the formation of the passive oxide film while the thickness of more than 0.3 p. not only prevents uniform electrodeposition of manganese dioxide but also makes undiscernible the indentations produced on the surface of the base substrate metal by the sandblast treatment.

As above mentioned the conventional electrodeposited manganese dioxide when used as an anode is liable to be detached from the substrate owing to the internal stress, and dissolved as permanganate ion into the electrolyte. In order to obviate such disadvantages the invention makes use of an aqueous solution of manganese sulfate containing 10-30 g./l. of free sulfuric acid as the composition of the electrolyte to be used in electrodepositing manganese dioxide and of current density of 0.0 l 5-0.007 A./dm.

The above-mentioned conditions for the electrodeposition renders it possible to obtain electrodeposited manganese dioxide which is very hard and dense and has less internal stress, thus improving the adherent property thereof. Moreover, the electrodeposited manganese dioxide has a small electric resistance which permits of decreasing the cell voltage during electrolysis. Moreover, the electrodeposited manganese dioxide is not dissolved into the electrolyte.

If the concentration of the free sulfuric acid lies outside of the above-mentioned range of 10-30 g.ll., the electrolytic voltage is gradually increased. When the concentration reaches more than 30 g./l., the electrodeposited manganese dioxide becomes detached from the substrate material. Moreover, when the current density becomes more than 0.05 A./dm. the cell voltage becomes high and the electrodeposited manganese dioxide is changed into permanganate ion which is soluble in the electrolyte. If the current density becomes too small, the electrodeposition of manfor practical use. The latter anode could be used as an insoluganese dioxide requires a plenty of times so that it is preferable anode for a considerably longer time, according to the ble to use the current density of more than 0.005 Ajdmf". conditions of electrodeposition.

It is desirable to make the thickness of the electrodeposited But, the latter electrode produced gradually a film having a manganese dioxide layer about 10-100 ,u.. If the thickness is 5 high electric resistance at the interface between the titanium larger than 100 n, the internal stress so increases as to cause substrate and the manganese dioxide layer to gradually inthe electrodeposited manganese dioxide to be detached from Crease the cell voltage. the substrate material, while ifthe thickness is smaller than The above-mentioned titanium plate whose surface was p, oxygen evolves on the urface of the thin layer which 1011- treated by sandblast to form minute indentations and the same t in a ba i itio l d f o a gro i ti f i0 titanium plate electroplated with a platinum layer having a platinum, palladium and rhodium, and forms on the surface of thickness f ywer electrodeposited with manganese he ba substrate meta] containing a b i composition dioxide under the electrodeposition conditions shown in the selected from a group consisting of titanium, zirconium d table I and the characteristics of the anodes thus obtained tantalum. This results in formation of a passive oxide film on were measured under h Conditions Shown in the table 2. he urfa e f th substrate t i l, 1 The result obtained were shown in table 4.

The optimum electrolytic conditions are the free sulfuric acid concentrations of -25 g./l. and the current density of TABLE 4 i Time used, Cell Volt- The anode of the invention satisfying the above optimum Anode s electrolytic conditions may be used in carrying out various Tttaniumplate having surfa e with minqte kinds of electrolysis without being desolved into electrolytes mdentatlons. a electrodoposmd manganese dioxide 500 3.2 to soil them. Even when the anode of the invention is dissolved Titanium plate having s irt acc with l'llllllllllu indentations an forme t iereon wit 1 p atrnto the electrolyte, the solute 15 not depos ted on the cathode mum layer and then electrodepositcd with so that it is possrble to obtain an electrolytic product having a manganese dioxide 2, 000 2.9 high purity.

The anode of the invention is low in the oxygen overvoltage during electrolysis so that the electrolysis can be carried out As seen from the above table 4, the titanium plate having without consuming rnuch electric power and, more particuthe surface with minute indentations and formed thereon with larly, can be applied to electrolytically obtain zinc containing the platinum layer and further electrodeposited with manlittle lead. ganese dioxide required lower cell voltage and were more The invention will now be described with reference to exdurable than the titanium plate having minute indentations on amples. the surface and directly electrodeposited thereon with man- A titanium plate having smooth surface whose thickness is l ganese dioxide.

and a ailabl in a k t and anoth titaniu lat iv n The relation between the concentration of free sulfuric acid sandblast treatment to form minute indentations on the urin the electrolyte used in the electrodeposition Of manganese face thereof were used as substrates, respectively. These dioxide and the anode characteristics is shown in table 5 and titanium plates with or without sandblast treatment were electhe relation between he rr nt density and the anode trodeposited with manganese dioxide by means of electrolysis chafaclel'lstlcs table The Samples were titanium Plates under h di i Shown i bl 1 to b i anodes, h 40 with minute indentations on the surface electroplated thereon characteristics of the anodes thus obtained were measured W 21 Platinum fy fi of P- and elemmdeposiled under the conditions shown in table 2. The results obtained Wnh manganese dloxlde- The anode Characteristics were are shown in table 3 sured under the conditions shown in the table 2.

TABLE 1 5 TABLE 5 Composition of electrolyte Conditions for electrodeposition Composition of I electrolyte, g./l. Anode characteristics Thickness Current of electro- Free density Manganese Temper- Current dflposlted 5O Manganese sulfuric clectrodepo- Cell 1 oltage lfate 2 4 allure densliy layer (33310116 sulfate acid sition, A./dm.- voltage, v. increase, v./day 150 g./l. 2o g./l. 90 0. 0.01 A./dm. 2o Pb 150 5 01 3.0 0. 03 150 10 0.01 2. 0.00 150 30 81 $3 38 150 0 1 a TABLE 2 150 0V 01 a. 1 0. 05

Composition of elplctrolyte Current dAepgity Tempera; 5 5

for zinc, in. true 0 g electro- TABLE 6 Zn H 804 Mn++ Atanode Atcathode 1yte C. Cathode Composition of 5045 150490 1 8 5 5. 5 35 A1 plate electrolyte, g./l. Anode characteristics Current 0 Free density TABLE 3 Manganese sulfuric elcctrodepo- Cell Voltagu sulfate acid sition, A./dm. voltage, v. increase, v./day Time used Cell volt- 1 150 .20 0. 01 .2. J 0. 00 Anode hrs. age, g 20 0.02 Titanium plate having smooth surface and 0 30 [L05 electrodeposition with manganese dioxide 16 4. 2 150 20 20 Titanium plate having minute indentations and electrodeposited with manganese dioxide 509 3. 2

As seen from the tables 5 and 6, it is necessary to use 10-30 I g./l. of the concentration of the free sulfuric acid in the elec- AS n r h Table h anode made of i ni m trolyte for use in the electrodeposition of manganese dioxide plate having the smooth surface and electrodeposited with and less than 0.05 A./dm. of the current density for the purmanganese dioxide was less durable and requires higher cell pose of obtaining the anode having excellent characteristics.

voltage than the anode made of the same titanium plate having A titanium plate, a zirconium plate, a tantalum plate and minute indentations on the surface and electrodeposited with Ti-1% Pd alloy plate, all having a thickness of 1 mm. and

manganese dioxide; so that the former anode is not suitable available in market, were used and were treated by sandblast on the surface. The plates thus treated were immersed in concentrated hydrochloric acid and then the surfaces thereof were made clean. Subsequently, these plates were covered with a layer of platinum, palladium or rhodium, each having a thickness shown in table 7. The layer thus formed was elecmainly been used as an anode for electrolytic winning of zinc,

with the result that the electrolytic zinc obtained contained comparatively large amount of lead as impurities. Thus, it was rather difficult to obtain zinc containing less than 0.003 percent oflead and adapted to be used for die casting.

On the contrary, the anode of the invention was used to win electrolytic zinc containing extremely small amounts of lead or substantially containing no lead, with the result that this electrolytic zinc obtained was suitable as die casting material The anode of the invention was used at the anodic potential of about 1.52 v. vs. S.C.E. (Standard Carmel Electrode) so that the cell voltage becomes less than about 2.9 v. On the contrary, the conventional lead alloy electrode was used at the anode potential of about 1.70 v. vs. S.C.E. so that the cell volt- 5 trodeposited with manganese dioxide under the conditions shown in table 8 to manufacture the anodes of the invention.

TABLE 7 Thickness of layer Covering process Pt Pd Rh Heat decomposition 0.02 0.03 Electrodeposition 0. 03 0. 05 0. l 0. 1

TABLE 8 Spacing between Composition of electrolyte electrodes Number (g./l.) Cathode (ern.) Conditions for electrolysis Current density, 0.007 A./dm. 1 {g 5 150 }lb 3 {Temperature of electrolyte, 00 C.

2 Thickness of electrodepositcd layer, 10 p.

, Current density, 0.01 AJdmfi. 2"... g 35 150 '}PB 3 {Temperature 01 electrolyte, 00 C.

2 Thickness of electrodeposited layer, 1.

Current density 0.015 A./dm. MI1S04'4H20 150 o 3 300" }Ph 3 {Temperature of electrolyte, 00 C.

Thickness of electrodeposited layer, 50

The anode thus obtained each showed a uniform elecage becomes about 3.2 v. This shows that the invention can trodeposition of manganese dioxide which was dense and had an excellent adhesive property and thus the manganese dioxide layer was not detached from the substrate material even by roughly handling.

The anodes manufactured as above described, when each used for electrolysis carried out with a cathode of a titanium plate in aqueous solution containing 3 percent salt water and 20 percent sulfuric acid at C. and the current density of 5 A./dm. withstood a long time of use without any defect.

A titanium plate was used and the surface thereof was treated by sandblast to form minute indentations thereon which were cleaned. The plate thus treated was electrodeposited with a platinum layer having a thickness of 005;. under the conditions shown in table 9 and then further electrodeposited with a manganese dioxide layer under the conditions shown in No. 2 of the table 8. The anode thus obtained was used under the conditions shown in table 10 to obtain electrolytic zinc.

TABLE 9 Composition of electrolyte (g./l.) Anode Condition for electrolysis Pt(NHa):'(NO2)2, 16.5.". o NH4NO ,100 Pt {Temperatureol e1ectr0lyte,80 C. 1i'aNO:,1O Current density, 2 AJdmJ.

Temperature of Electrolyte,

Duration of electrolysis, hours 24 Spacing between electrodes, 3

H2304 Mn ion 1 Highly pure aluminum.

The zinc obtained was highly pure zinc containing less than 0.001 percent of leadv Heretofore, Pbl% Ag alloy has not only economize the electric power consumption but also ensure reduction of the rate of oxidation of manganese ion in the electrolyte at the anode owing to the fact that the anodic potential is low, thereby reducing the amount of manganese oxide crust to be produced. Moreover, the anode of the invention can be used as an anode for use in zinc electrolysis for a long time in a stabilized state since manganese ion present in the electrolyte serves to prevent wear of the manganese dioxide layer.

It will be understood that the invention is not limited to the examples described and that many modifications may be introduced therein without departing from the scope of the invention.

We Claim:

1. A method of manufacturing an insoluble anode for use in electrolytic winning of zinc, which comprises treating by sand blast a base substrate metal of titanium or its alloy to form minute indentations on the surface, electroplating on said base substrate metal a thin layer of platinum, and electrodepositing a layer of manganese dioxide on said thin layer in aqueous solution of manganese sulfate containing 10-30 g./l. of free sulfuric acid at a current density of0.0l5-0.007 A./dm.

2. A method of manufacturing an insoluble anode for use in electrolytic winning of zinc in accordance with claim 1 wherein the concentration of free sulfuric acid in said aqueous solution of manganese sulfate is l525 g./l. and said current density is 005-0005 A./dm.

3. An insoluble anode suitable for use in electrolysis which comprises three components a. a base substrate treated by sandblasting, said substrate being selected from the group consisting of titanium, zirconium and tantalum or alloys thereof;

b. a thin metal layer formed on said substrate, said thin metal layer being selected from the group consisting of platinum, palladium and rhodium or alloys thereof; and

. an outermost layer of manganese dioxide electroplated on said thin metal layer in an aqueous solution of manganese sulfate containing about 10 to about 30 g./l. of free sulfuric acid at a current density of about 0.015 to about 0.007 A./dm..

4. The insoluble anode of claim 3 wherein the thickness of said thin metal layer (b) is 0.01 to 0.3 u and the thickness of said manganese dioxide layer (c) is 10 to p.. 

2. A method of manufacturing an insoluble anode for use in electrolytic winning of zinc in accordance with claim 1 wherein the concentration of free sulfuric acid in said aqueous solution of manganese sulfate is 15-25 g./l. and said current density is 0.05-0.005 A./dm.2.
 3. An insoluble anode suitable for use in electrolysis which comprises three components a. a base substrate treated by sandblasting, said substrate being selected from the group consisting of titanium, zirconium and tantalum or alloys thereof; b. a thin metal layer formed on said substrate, said thin metal layer being selected from thE group consisting of platinum, palladium and rhodium or alloys thereof; and c. an outermost layer of manganese dioxide electroplated on said thin metal layer in an aqueous solution of manganese sulfate containing about 10 to about 30 g./l. of free sulfuric acid at a current density of about 0.015 to about 0.007 A./dm.2.
 4. The insoluble anode of claim 3 wherein the thickness of said thin metal layer (b) is 0.01 to 0.3 Mu and the thickness of said manganese dioxide layer (c) is 10 to 100 Mu . 